[whatwg/dom] Export next and previous sibling? (#791)

The [ContentEditable spec](https://w3c.github.io/contentEditable/) currently talks about the {{Node/nextSibling}} IDL attribute, but I think it really wants to "next sibling" and "previous sibling" concepts, which are not exported from DOM. 

Should we export those concepts? Happy to send a PR, but wanted to check first. 

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